The latest lithography for nanofabrication in, for example, semiconductors, optical devices, and magnetic devices requires patterning precision on the order of tens of nanometers or less. To achieve such high-precision patterning, intensive development has been made in various fields such as light sources, resist materials, and steppers.
Effective approaches to enhance the dimensional precision of nanofabrication include use of shorter wavelengths and convergent electron or ion beams in an exposure source. However, short-wavelength exposure sources and convergent electron or ion beam irradiation sources are so expensive that these sources are unsuitable for providing less expensive devices.
To enhance the dimensional precision of machining with the same exposure source as that in exposure apparatuses currently in use, other approaches have been proposed such as improvements in illumination methods and use of a special mask referred to as a phase shift mask. Further approaches have been attempted which include methods with a multilayer resist or an inorganic resist.
An exposure method has generally been employed which involves organic resists such as novolac resists and chemically amplified resists with ultraviolet light as an exposure source. Organic resists, which are versatile, have extensively been used in the field of lithography. However, their large molecular weight results in an unclear pattern at the boundary between exposed and unexposed areas. This is disadvantageous from the viewpoint of enhancing the precision of nanofabrication.
In contrast, inorganic resists, which have a low molecular weight, provide a clear pattern at the boundary between exposed and unexposed areas, and have the possibility of achieving high-precision nanofabrication compared to organic resists. For example, Jpn. J. Appl. Phys., Vol. 30 (1991), p. 3246 introduces a nanofabrication method with, for example, MoO3 or WO3 as a resist material and ion beams as an exposure source; and Jpn. J. Appl. Phys., Vol. 35 (1996), p. 6673 introduces a method with SiO2 as a resist material and electron beams as an exposure source. Furthermore, SPIE, Vol. 3424 (1998), p. 20 introduces a method with chalcogenide glass as a resist material and 476 and 532 nm lasers and ultraviolet light from a mercury-xenon lamp as exposure sources.
The use of electron beams as an exposure source can be combined with many kinds of inorganic resist materials, as described above, while only chalcogenide has been reported as a material corresponding to ultraviolet or visible light. The reason is that inorganic resist materials proposed other than chalcogenide that are transparent to ultraviolet or visible light have significantly low absorbance, which is unsuitable for practical use.
Chalcogenide has the advantage of allowing ultraviolet or visible light and therefore exposure apparatuses currently in use, but has the problem of containing some agents harmful to humans, such as Ag2S3, Ag—As2S3, and Ag2Se—GeSe.
On the other hand, photolithography with ultraviolet or visible light are extensively applied to the manufacture of various devices such as semiconductor devices, such as dynamic random access memory (DRAM), flash memory, central processing units (CPUs), and application specific ICs (ASICs); magnetic devices, such as magnetic heads; displays, such as liquid crystal displays, electroluminescent (EL) displays, and plasma display panels (PDPs); optical devices, such as optical recording media and optical modulation elements. Examples of these devices are compact discs (CD, which is a registered trademark), which are read-only optical discs as typified by DVDs. The structure of an optical disc will be described below.
An optical disc essentially includes an optically transparent substrate of, for example, polycarbonate, which has a main surface with a fine irregular pattern of, for example, pits and grooves representing information signals. The main surface is covered with a thin reflective film of a metal such as aluminum, which is further covered with a protective film.
Such a fine irregular pattern on the optical disc is formed with a stamper having a high-precision fine irregular pattern through a process of transferring the pattern onto the substrate faithfully and readily. A method for preparing the stamper will be described below.
For example, a glass substrate with a sufficiently smooth surface is disposed on a rotating platform. A photoresist, which is photosensitive, is applied onto the glass substrate rotating at a predetermined rotational speed. The rotation spreads the photoresist over the glass substrate, so that the glass substrate is entirely spin-coated. The photoresist is exposed to recording laser light in a predetermined pattern to form a latent image corresponding to information signals. The photoresist is then developed with a developer to remove an exposed or unexposed area, thereby providing a resist master with the predetermined irregular pattern of the photoresist. Metal is further deposited on the irregular pattern of the resist master by a process such as electroplating to transfer the irregular pattern to the metal. The metal, which is a stamper, is separated from the resist master.
The stamper is used to duplicate a large number of substrates made of thermoplastic resin, such as polycarbonate, by known transferring processes such as injection molding. Each of the substrates is then covered with, for example, a reflective film and a protective film to complete an optical disc.
The capacity of information recordable on the optical disc depends on the density of pits or grooves that can be formed. In other words, the capacity of information recordable on the optical disc depends on the fineness of the irregular pattern formed by cutting, namely, exposing a resist layer to laser light to form a latent image.
For example, a stamper for read-only DVDs (DVD-ROMs) has a spiral pit string with a minimum pit length of 0.4 μm and a track pitch of 0.74 μm. An optical disc 12 cm in diameter, produced with a stamper as a mold, has an information capacity of 4.7 GB per side.
Production of optical discs with such a structure requires a resist master prepared by a lithography process using a laser with a wavelength of 413 nm and an objective lens with a numerical aperture (NA) of approximately 0.90 (for example, 0.95).
With the current rapid progress in information and communication technology and image-processing technology, optical discs as described above are facing the task of achieving a recording capacity several times higher than the current capacity. For example, next-generation optical discs with a diameter of 12 cm, an extension of digital videodiscs, are required to attain an information capacity of 25 GB per side by conventional signal processing. To meet this requirement, the minimum pit length and track pitch of the optical discs must be reduced to approximately 0.17 μm and 0.32 μm, respectively.
The minimum pit length P (μm) in exposure is represented by equation (1) below:P=K·λ/NA   (1)where λ (μm) represents the wavelength of the light source; NA represents the numerical aperture of the objective lens; and K represents a proportional constant.
The wavelength λ of a light source and the numerical aperture NA of an objective lens are parameters depending on the specification of a laser apparatus. The proportional constant K is a parameter depending on a combination of the laser apparatus and the resist layer.
In the production of the above optical discs, such as DVDs, setting the wavelength to 0.413 μm and the numerical aperture NA to 0.90 leads to a minimum pit length of 0.40 μm, then providing a proportional constant K of 0.87 from equation (1) above.
In general, a shorter laser wavelength is effective to achieve the fine pit described above. That is, in the case of the same proportional constant K and, for example, NA=0.95, a light source with a laser wavelength λ of 0.18 μm is required to provide the minimum pit length of approximately 0.17 μm, which is necessary for high-density optical discs with a recording capacity of 25 GB per side.
The wavelength of 0.18 μm required in this case is shorter than a wavelength of 193 nm of an ArF laser, which is being developed as a light source for next-generation semiconductor lithography. An exposure apparatus achieving such a short wavelength requires special optical components, such as the lens, as well as a special laser as a light source, thus becoming extremely expensive. In increasing the optical resolution to address nanofabrication, an approach based on a shorter exposure wavelength λ and a larger numerical aperture NA is quite unsuitable for production of inexpensive devices due to the following reason: this approach inevitably requires the replacement of the exposure apparatuses currently in use with expensive exposure apparatuses because the exposure apparatus in use cannot keep up with the advances in nanofabrication.
The present invention is proposed to solve such conventional problems. An object of the present invention is to provide a resist material allowing high-precision nanofabrication without an expensive irradiating apparatus using, for example, electron beams or ion beams. Another object of the present invention is to provide a nanofabrication method allowing finer processing with exposure apparatuses currently in use and the resist material.